The present invention relates to a selectively oxygen-permeable substrate, a metal-air battery positive electrode, and a metal-air battery. More specifically, the present invention relates to a selectively oxygen-permeable substrate capable of selectively introducing oxygen in the air into the inside and having high durability against an electrolytic solution. In addition, the present invention relates to a metal-air battery positive electrode provided with the selectively oxygen-permeable substrate and to a metal-air battery provided with the metal-air battery positive electrode.
Since the metal-air battery employs metal as the negative-electrode active material and oxygen as the positive-electrode active material, the battery has extremely large discharge capacity per “unit mass of the positive-electrode active material”. In recent years, increased capacity and increased output of the battery have been required in electric automobiles, portable appliances, and the like, and improvement in performance of the metal-air batteries is expected.
As described above, since the positive-electrode active material of a metal-air battery is oxygen, it is possible to use oxygen in the air as the positive-electrode active material. However, when the air is introduced into the metal-air battery as it is, also carbon dioxide in the air is introduced. Since carbon dioxide reacts with an electrolytic solution used in the metal-air battery, and battery performance may be deteriorated, it is preferable to inhibit carbon dioxide from being introduced into the metal-air battery. Thus, in the case of using oxygen in the air as the positive electrode for a metal-air battery, there has been a problem that carbon dioxide is also supplied to the metal-air battery though it is preferable to inhibit carbon dioxide from being introduced into the positive electrode of the metal-air battery.
In order to solve such a problem, there is investigated a method of removing carbon dioxide from the air supplied to a metal-air battery (see, e.g., JP-A-10-99629).
On the other hand, there is investigated a selectively oxygen-permeable membrane (oxygen enrichment membrane) capable of concentrating oxygen in the air and having magnetic particles dispersed therein (see, e.g., JP-2007-237138).
In the metal-air battery described in JP-A-10-99629, a gas separator provided with a ferrite magnet and a stainless steel is used as a device for selectively permeating oxygen. Therefore, it is imagined that the entire metal-air battery has a large volume and mass.
The oxygen enrichment membrane described in JP-A-2007-237138 is a silicone based membrane. In an organic polymer membrane of silicone or the like, since gas permeation is performed by a dissolution diffusion mechanism derived from a flexible molecular chain, dissolution of carbon dioxide in the membrane cannot be inhibited. Therefore, if a silicone based membrane is used as an oxygen concentration membrane upon using oxygen in the air as the positive-electrode active material of a metal-air battery, it is imagined that sufficient oxygen concentration cannot be performed due to dissolution of carbon dioxide in the membrane. It is also imagined that a silicone based membrane does not have sufficient durability against an electrolytic solution used in the metal-air battery.